Chapter 8

Chapter 8 – Subroutines and Control
Abstractions
1
Three major parts of a runtime
environment:
• Static area allocated at load/startup time. Examples:
global/static variables and load-time constants.
• Stack area for execution-time data that obeys a lastin first-out lifetime rule. Examples: method variables
and temporaries.
• Heap or dynamically allocated area for "fully
dynamic" data, e.g. data allocated with new
2
Procedure Overview
• When functions are “first class” data items themselves, they can be
dynamically created and used like values just like any other data
structure. (e.g., Haskell curried functions, eval)
• pass functions as arguments
• A procedure is called or activated.
• Activation record: collection of data needed to maintain a single
execution of a procedure.
• access to local and non-local references.
• Static or dynamic environment (depending on scoping) must be
accessible during runtime.
• When a procedure depends only on parameters and fixed language
features – closed form.
• The code for a function together with its defining environment is called
closure – as we can resolve all outstanding non-local environments.
3
Implementing “Simple” Subprograms
• Caller responsibilities
1. Save the execution status of the caller (calling
environment)
2. Carry out the parameter-passing process by putting the
parameters somewhere that the called function can
access.
3. Pass the return address of the caller to the callee
4. Transfer control to the callee
4
Implementing “Simple” Subprograms
• Return Actions:
1. If it is a function, move the return value to a place the
caller can get it
2. Restore the execution status of the caller
3. Transfer control back to the caller
5
Implementing “Simple” Subprograms
Called routine must
• Create an activation record:
–
–
–
–
local variables
return address
points to other environments
parameters
– See CSILM lesson “Call Stack” for an example
csilm.usu.edu, Browse Resources, CSILM Activities,
Programming Languages, Call Stack
6
Parameter Passing
• Aliases may be created
• Type checking parameters – for legality and to pick
between overloaded methods
• a reference parameter is a nonlocal variable
• the same data object passed for two parameters
CALL S(X,X) causes aliasing
• With aliasing, interesting problems in optimizations
occur.
x+2
y++
x+2
If x and y are aliases, x+2 isn’t a common
subexpression
7
Models of Parameter Passing
8
• 1. Pass-by-value (in mode)
– Typically we copy the value in, but can be done with a
constant reference pointer.
– Parameters are viewed as local variables of the procedure
– Disadvantages of copy:
• Requires more storage (duplicated space)
• Cost of the move (if the parameter is large)
– Disadvantages of implementing by constant reference:
• Must write-protect in the called subprogram or compiler check that
there are no assignments.
• Accesses cost more (indirect addressing)
9
2. Pass-by-result (out mode)
–
–
–
–
function return value(s)
Local’s value is passed back to the caller
Physical move is usually used
Disadvantages:
• If value is passed, time and space costs to copy
• order dependence may be a problem (if output values are aliased)
procedure sub1(y: int, z: int);{
y=0;z=5;
}
sub1(x, x);
– Value of x in the caller depends on order of assignments at the return
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3. Inout mode
– Pass by value-result (aka copy-in copy-out or
copy-restore)
– Used to save cost of indirect access.
Physical move, both ways
– Disadvantages
– ordering may be a problem with a call like doit(x,x)
– time/space issues
– Need to know whether address is computed again before
copying back. doit(i,a[i])
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4.Pass by reference.
Issues:
• passing is faster (as no data copy)
• formal parameter is local object of type pointer
• If expression is passed as an in/out parameter: a
temporary location may be passed
(and then the copy is changed, not the original)
• Disadvantages:
– access slower as is indirect (always follow a
pointer to access), but passing is fast (only copy a
pointer, not a whole structure)
– may make inadvertent changes to parameters
12
5. Pass-by-name (Delayed evaluated parameters)
– By textual substitution
– parameter is evaluated everytime it is used, but it is
evaluated in the caller’s environment
– Purpose: flexibility of late binding
– costly
Thunks: a pass by name argument is
implemented by a little procedure
(called a thunk) which evaluates the
argument.
A thunk is a method to evaluate an expression that is yet to
be evaluated.
13
Pass-by-name
How is it like other methods?
• If actual is a scalar variable, it is pass-by-reference
• If actual is a constant expression, it is pass-by-value
• If actual is an array element, it is like nothing else
e.g.
procedure sub1(x: int; y: int);
begin
x := 1;
Seems like nothing is happening
y := 2;
with first assignments but it is
x := 2;
y := 3;
end;
sub1(i, a[i]);
14
• Example:
procedure R(var i,j: integer);
begin
m:boolean;
m := true;
i++;
j++;
end;
m := 2;
R(m,c[m]);
• pass by reference: adds 1 to m and c[2]
Pass by name: adds 1 to m and c[3]
15
Parameter Passing Methods
• Design Considerations for Parameter Passing
1. Efficiency
2. One-way or two-way
– These two are in conflict with one another!
– Good programming => limited access to variables,
which means one-way whenever possible
– Efficiency => pass by reference is fastest way to pass
structures of significant size
– Also, functions should not allow reference parameters
– Look at csilm.usu.edu Programming/
PROGRAMMING LANGUAGES/Parameter Passing
16
Languages and Environments
• Languages differ on where activation records are in
the environment:
– Fortran is static: all data, including activation records, are
statically allocated. (Each function has only one activation
record—no recursion!)
– Functional languages (Scheme,ML) and some OO
languages (Smalltalk) are heap-oriented: all (or almost all)
data, including activation records, are allocated
dynamically.
– Most languages are in between: data can go anywhere
(depending on its properties); activation records go on the
stack.
17
Simple stack-based allocation
• Nested declarations are added to the stack as their
code blocks are entered, and removed as their code
blocks are exited.
• Example:
Stack at Point 1:
{ int x; int y;
{ int z;
w
}
y
{ int w;
x
// Point 1
}
}
• Note ,z has been removed at point 1
as have exited scope
18
Example (C): main →q →p
int x;
void p( int y)
{ int i = x;
char c; ...
}
void q ( int a)
{ int x;
p(1);
}
main()
{ q(2);
return 0;
}
19
Local variable access using the ep
• In a typical language with a stack-based runtime
environment, the local declarations in a procedure are fixed
at compile-time, both in size and in sequence.
• This information can be used to speed up accesses to local
variables, by precomputing these locations as offsets from
the ep.
• Then the local frame need not have a name-based lookup
operation (unlike the symbol table).
• In fact, names can be dispensed with altogether.
20
Non-local variable access
• Requires that the environment be able to identify frames
representing enclosing scopes.
• Using the dynamic link results in dynamic scope (and also
kills the fixed-offset property as you are not sure which
method will contain the x. Thus, you can’t depend on a
fixed location).
• If procedures can't be nested (C, C++, Java), the enclosing
scope is always locatable by other means: it is either global
(accessed directly) or belongs to the current object.
• If procedures can be nested, to maintain lexical scope a
new link must be added to each frame: the static link,
pointing to the activation of the defining environment of
each procedure.
21
Nested Subprograms
• The process of locating a nonlocal reference:
1. Find the correct activation record instance
2. Determine the correct offset within that activation
record instance
May need to follow several links (static chaining)
The number of links is known from compile time.
If used stack of symbol tables, can count how many
tables you had to search to find it.
If used individual stacks for each value, you can
record the nesting depth of each variable.
22
Procedure values as pointer pairs
• Each procedure becomes a pair of pointers: a code pointer
(called the instruction pointer or ip), and an environment
pointer (ep) pointing to the definition environment of the
procedure (which will become the access link during a
call).
• Such an <ep,ip> pair is sometimes called a closure.
• In computer science, a closure is a function together with a
referencing environment for the nonlocal names (free
variables) of that function. Such a function is said to be
"closed over" its free variables.
23
The Process of Locating a Nonlocal
Reference
• Finding the offset is easy
• Finding the correct activation record instance:
– Static semantic rules guarantee that all nonlocal
variables (that can be referenced) have been allocated in
some activation record instance that is on the stack
when the reference is made
24
Nested Subprograms
• Static Chain Maintenance
– At the call :
• The activation record instance must be built
• The dynamic link is just the old stack top pointer
• The static link must point to the most recent ari of the static
parent (in most situations)
– Two Methods to set static chain:
1. Search the dynamic chain until the first ari for the static
parent is found--easy, but slow
25
Nested Subprograms
2. Treat procedure calls and definitions like variable
references and definitions (have the compiler compute
the nesting depth, or number of enclosing scopes
between the caller and the procedure that declared the
called procedure; store this nesting depth and send it
with the call)
26
Nested Subprograms
• Evaluation of the Static Chain Method
– Problems:
1. A nonlocal reference is slow if the number of scopes
between the reference and the declaration of the
referenced variable is large
2. Time-critical code is difficult, because the costs of
nonlocal references are not equal, and can change
with code upgrades and fixes
27
Nested Subprograms
•
Technique 2 (for locating non-local variables
with static scope) - Displays
–
The idea: Put the static links in a separate stack called
a display. The entries in the display are pointers to the
ari's that have the variables in the referencing
environment.
– Represent references as
(display_offset, local_offset)
where display_offset is the same as chain_offset
– Can access via computation. display offset of 10 is
one lookup (not a chain of length 10)
28
Main – level 0
p
p level1
t level 1
q level 3
s level 2
r level 2
29
100
Stack is shown growing downwards.
Display contains pointers to each
activation record at each reachable level
main
200
t
100
Level 0
200
Level 1
300
Level 2
400
Level 3
300
s
400
main-> t -> s-> q
q
When s calls q, a single element is added to the
table.
30
At your seats…
• Why do we do this?
• This seems well and good – but how do we keep it
current?
31
100
main
200
t
100
Level 0
500
Level 1
300
Level 2
400
Level 3
300
s
q
old level 1 is 200
400
500
p
main-> t -> s-> q-> p
When q calls p, a new level 1 entry is needed.
Store the old one, so you can get it back.
Level 2 and level 3 are unused (but unchanged)
32
100
main
200
t
100
Level 1
600
Level 2
300
Level 3
400
Level 4
300
s
q
old level 2 is 200
p
old level 2 is 500
t
400
500
main-> t -> s-> q-> p->t
When p calls t, a new level 2 entry is needed
Level 3 and level 4 are unused (but unchanged)
600
33
100
main
200
t
100
Level 1
600
Level 2
300
Level 3
400
Level 4
300
s
q
old level 2 is 200
p
old level 2 is 500
t
400
500
main-> t -> s-> q-> p->t
When p calls t, a new level 2 entry is needed
Level 3 and level 4 are unused (but unchanged)
600
34
Implementing Dynamic Scoping
1. Deep Access (search) - nonlocal references are
found by searching the activation record instances
on the dynamic chain
– Length of chain cannot be statically determined
– Every activation record instance must have variable
names recorded
35
Implementing Dynamic Scoping
2. Shallow Access - put locals in a central place
• How implemented?
a. One stack for each variable name
b. Central referencing table with an entry for each variable
name
At subprogram entry, add location for each variable.
At subprogram exit, remove location for each variable.
36
Using Shallow Access to Implement
Dynamic Scoping
37
Parameter Passing Conventions
• Actual/Formal Parameter Correspondence:
1. Positional (this is what we are used to)
2. Keyword
– e.g. SORT(LIST => A, LENGTH => N);
– Advantage: order is irrelevant
– Disadvantage: user must know the formal parameter’s names
3. Default Values
38
Overloaded Subprograms
• Def: An overloaded subprogram is one that has the
same name as another subprogram in the same
referencing environment
• C++ and Ada have overloaded subprograms builtin, and users can write their own overloaded
subprograms
• Overloaded subprograms provide ad hoc (nongeneralizable) polymorphism
39
Generic Subprograms
• Analogy: Would you rather have:
– Vacuum that can adjust to any carpet height versus
– A different vacuum for each type of carpet.
• A generic or polymorphic subprogram is one (not
many) subprogram that takes parameters of different
types on different activations
• A subprogram that takes a generic parameter needs to
figure out what type was passed (Haskell)
40